ECU

• Vehicles possess an engine control unit, which uses sensors to monitor numerous functions that take place that enable the vehicle to operate. The ECU monitors idle speed by either reading ignition pulses that are produced by the ignition module in the distributor, or by signals from the throttle switch or position sensor. These signals tell the ECU if the throttle is closed and the engine is idling.
  
  

IAC Valve

• When the engine's idle increases or decreases to a preset level, the computer signals the IAC solenoid to either open or close the IAC valve. The IAC valve is on the throttle body and is designed to regulate engine idle speed.
  
  

Throttle Bypass

• The IAC valve opens a bypass circuit, which enables air to bypass the throttle. This in turn increases idle speed. Closing the bypass circuit reduces the engine's idle speed.